PLANT UPGRATABILITYAir Liquide indicated that the Air Separation plant can be uprated to:-

Nitrogen output = 8,600 Nm3/hrOxygen output = 4,200 Nm3/hr

Process Description:

Air separation is a cryogenic process, where atmospheric air is cooled to low temperatures so that the gases liquefy and final separation is achieved in a double distillation column. Five basic processes are used to achieve separation of air to obtain pure nitrogen and oxygen. These include compression, purification, heat exchange, liquefaction and distillation

Compression

Atmospheric air is drawn in at the suction point and is filtered by a double staged filter so as to avoid fouling of process equipment by impurities such as leaves, grass, dust and insects. Filtered ambient air from the suction enters the BBC unit (centrifugal compressor) where air is compressed in a series of five stages. This unit is incorporated with inter-stage coolers so as to remove the heat of compression. Compressed air is delivered at 75°C and 4.5 bar pressure at the discharge and is cooled in a final water cooler to 30°C resulting in condensation of moisture. The formed water droplets leave the unit via water trap drains. This process air is delivered to the refrigeration unit.

Refrigeration

The refrigeration unit consists of a shell and tube heat exchanger (ammonia evaporator), two Quiri compressors, ammonia condenser and an oil separator. The compressed air stream air stream is cooled to 5°C in the shell and tube heat exchanger resulting in the moisture in the air to condense. Ammonia is used as a refrigerant in the heat exchanger. Any entrainment of water droplets in air stream is captured by the mist pad in a water separator as the air exits the refrigeration unit.

Purification

The compressed air stream is purified in the drying and carbon dioxide removal unit where residual moisture and carbon dioxide is removed from air stream. The drying units comprise of two bottles which contain molecular sieve material which adsorbs moisture and carbon dioxide. These impurities need to be removed since they freeze out at low temperatures (moisture and carbon form ice and dry ice at 0°C and -78°C respectively) resulting in blockage, inefficient heat exchange and fouling of equipment in the downstream processes. Dry, carbon dioxide free gas is forwarded to the plate heat exchangers.

Heat exchange

Compressed air from the drying unit enters the plate heat exchangers and heat exchange takes place between this air stream and product streams (pure nitrogen, oxygen and impure nitrogen) exiting the distillation column. At the base of the plate heat exchangers air at -172°C is forwarded to the medium pressure column of the double distillation column. A portion of the compressed air at -140°C is taken out 2/3 down the exchangers and is delivered to the turbine expanders.

Double distillation column

The double distillation column consists of two compartments separated by a vaporizer at the center of the tower. The low pressure column is positioned at the top with an optimum operating pressure of 0.5 bars and the medium pressure column is at the bottom operating at 4.5 bar pressure.

Liquefaction

Turbine expanders provide the required refrigeration to produce liquids in the distillation column. Two expansion turbines, one of which is capable of keeping the plant on full production rates, are provided and external work recovered in the form of a generator coupled to the turbine to produce electrical power which is fed into the plant’s 380V distribution system. This transfer of energy causes the gas to cool from -140 to -170°C and pressure drops from 4.5bars to 0.45bars. This stream of air passes to the low pressure column where final separation is achieved.

Distillation

Separation is achieved over the 35 perforated trays of the medium pressure column. In order to remove any acetylene which may have accumulated in the liquid oxygen, the liquid oxygen in the vaporizer is circulated through liquid oxygen filters containing Mobil Sorbead (a silica gel) which removes any traces of hydrocarbons present in the liquid. Pure nitrogen gas (99.99% purity) and liquid oxygen are the products from the distillation process. Gaseous oxygen produced is vented to the atmosphere and the nitrogen is forwarded to the ammonia synthesis plant.